The abdomen is our physical centre, the core of our being. When we curl up in foetal position, we are protecting our vital organs, as opposed to our head. The abdomen is a place that is intricately linked to our emotions. This is the place where our emotions are felt and expressed.

When we have a strong instinct, we call it a gut feeling. When some one is willing to take a risk, we say they “have guts”. As Stephen Colbert says: “This is where the truth comes from – the gut. Facts come from the brain – and some people think that makes facts better. But did you know you have more nerve endings in your stomach than in your brain… You can look it up.”

Technically known as “the enteric nervous system” (ENS), our guts begin at the esophagus and ending at the anus, and are lined with sheaths of neurons, up to 100 million neurons – less than the brain, but more than either the spinal cord or the peripheral nervous system.

The function of this nervous system is the daily grind of digestion: breaking down food, absorbing nutrients and expelling waste requires chemical processing, mechanical mixing and rhythmic contractions to keep everything moving in the right direction.

Evolutionary wise, this abdominal brain is our primary one. The most primitive nervous systems were seen in simple tubular animals, which stuck to rocks on the sea floor and waited for food to float by. As life evolved, animals required a more complex brain and so developed a central nervous system (CNS), capable of movement, procreation, and assessment. Nature did not combine the two nervous systems – but rather preserved the Enteric nervous system as an independent circuit fully active from birth and mostly functioning without instruction from the Cerebral brain.

The two nervous systems are formed out of the same tissue during development: a clump of tissue called the neural crest forms early in embryonic genesis. One clump turns into the CNS, another piece migrates down to become the ENS. Only later do the two become connected by the vagus nerve.

Suprisingly, 90% of fibres in the vagus nerve carry information from the gut to the brain, not the other way round. The abdomen informs our state of mind in other ways too – and it is likely that our emotions are influenced by nerves in our gut.

Researchers at McMaster University have conclusive evidence that bacteria residing in the gut influence brain chemistry and behaviour. The gut is home to about 1,000 trillion bacteria with which we live in harmony. These bacteria perform a number of functions vital to health: they harvest energy from the diet, protect against infections and provide nutrition to cells in the gut.

Previous studies have suggested gut bacteria may communicate with the brain. For instance, some people with liver disease experience changes in mental abilities that improve after they are given antibiotics.

To further investigate the link, researchers gave healthy adult mice antibiotics to disturb their natural gut bacteria. The mice become less anxious and more eager to explore. This also appears to affect brain chemistry: mice had an increased amount of a brain protein called brain derived neurotrophic factor (BDNF). Changes in the levels of BDNF have previously been linked to depression and anxiety. When oral antibiotics were discontinued, bacteria in the gut returned to normal. This was accompanied by restoration of normal behaviour and brain chemistry.

Next the researchers carried out some gut bacteria swapping. Different strains of mice are known to exhibit different behaviour patterns. Some are more anxious while others are aggressive and hyperactive. The researchers took mice from both extremes and exchanged their gut bacteria. The behaviour flipped with the new bacteria: aggressive mice became passive and vice versa.

Researchers suspect the bacteria are producing chemicals that can access and influence the brain.

It is possible therapies that aim to restore normal gut flora, such as using probiotics, may be helpful not only in gastrointestinal disease, but also correct behaviour and mood changes in frequently associated diseases such as anxiety, depression, and late onset autism.

This brings us to examine emotions – and where they come from. One tends to assume that an emotion is experienced in the brain, and afterwards we have corresponding body changes: we are embarrassed, and so we blush, for example. But changing bodily states may trigger our emotions also: our hearts race causing us we feel elated. Like Siamese twins, the mind and body are interconnected: when one changes the other does too.